[WIP] SciDB backend

This isn't ready to merge, but I've started playing around with connecting SciDB to blaze.

There are two failing tests currently (I think one is a backend bug, and one is a SciDB bug). I also want to add some more tests to make sure I understand how blaze is working.
I expect it'll take us a week or so.  In any event it's a big enough internal-API break that I'm happy to do the work on PRs like this one.